Method and a device for press casting

ABSTRACT

A liquid metal (49) poured into a filling chamber (20) is influenced in this chamber by a lower piston (43) and an upper piston (18), which are mounted on a lower machine table (5) and an upper machine table (10) respectively, which tables (5 and 10 respectively) support a lower mold part (24) and an upper mold part (22) respectively. The filling chamber (20) is disposed in or at the lower mold part (24) and receives, as regards the poured-in amount of metal, the lower piston (43), which is adjustable as to its elevational level and can be adjusted to make the amount of metal reach a level somewhat below the mold cavity (51) between the mold parts (22, 24). The lower piston (43) has a larger diameter than the upper piston (18), which, in connection with the filling of the metal (41), is designed to be lowered to a central position in a guiding and feeding sleeve (19) in the upper mold part (22) and is retained in this position by an adjustable holding force. The lower piston (43) is subsequently designed to be pressed upwards and carry out the basic feeding into the mold cavity (51), counteracted up to the holding force by the upper piston (18), which is designed to, in the final phase of the basic feeding, be actuated by a larger force and be lifted somewhat to an upper holding position. The upper piston (18) is designed to, subsequent to the pressing movement of the lower piston (43), be activated and lowered to a lower position, the final pressing being done, subsequent to which the mold parts (22, 24) are designed to be opened up and to release the casting.

The present invention relates to a method of press casting. Theinvention also relates to a device, designed for press casting.

In order to in a rational way utilize the capacity of a press castingmachine it is in most cases used to cast different objects. In order tocast these different objects, i.e. to change from a casting of a certaintype of object to a certain other type of object, the lower piston aswell as the upper piston are usually exchanged as well as the sleevewhich constitutes the filling chamber and of course the mold as well asthe sleeve, which surrounds the lower piston. These measures are ofcourse always time-consuming and expensive processes and thereforeobjectionable. Also, the casting process is made more difficult in thisway, since e.g. constantly changed diameters of casting pistons, used alittle while, lead to varying pressures, which in practice is notacceptable on indeed not feasible. Consequently, a change as regards thecasting piston diameter, i.e. an exchange of a casting piston, must beaccompanied by a corresponding change of the compression pressure of themachine. This implies automatically a substantial risk of having thepersonnel make miscalculations or wrong adjustments of the pressure,resulting in a worse quality or actually a rejection of castings.

Particularly when the casting is done under relatively low pressures,e.g. between 1 and 200 bars, a few problems, so far unsolved, willappear. The rationale of such a low pressure casting is to make possiblea use of sand mold cores, which in its turn makes possible a productionof more complicated products or products, which are to meet high qualityrequirements, e.g. car rims, sand mold cores however not beingabsolutely required. The particular complex of problems of a lowpressure casting comprises partly the necessity of continuously usingstrongly heated molds and a superheated metal and partly prolonged cycleperiods, since the solidification times are absurdly prolonged, e.g.between 100 and 300 seconds. Thanks to the comparatively slowsolidification the metal structure will be coarse and have a lowstrength. In case a blacking occurs, it is true that it is advantageousduring the filling of the liquid metal, but it is a drawback during theensuing solidification, because it prolongs the cycle periods andcontributes to a larger grain size and hence a coarser structure and alower strength. Thus, it has so far been necessary in many cases tooversize the products.

JP 59-130688 relates to already known techniques in this field. Thelower piston according to this specification has a larger diameter thanthe upper piston, but no specific reasons or explanations are mentionedwhy this is or must be the case. The activation of the initiation of themovement of the upper piston is done, when a pressure influence has beenobtained from the melt via the movement upwards of the lower piston. Asregards the lower piston only a general movement upwards is mentioned.As regards the upper piston only a general movement downwards ismentioned.

The object of the present invention is to, in the above-mentionedrespects, improve and develop the already known method and devices ofpress casting. Also, another object is to, in this field, develop thestate of the art in various respects and create opportunities of arationally useful technique and products having a high and uniformquality.

One particular object of the present invention is to improve the castingwhen relatively low pressures are used by achieving shorter cycleperiods, improved product quality, particularly as regards a higherstrength, a lower porosity and a finer material structure, namely amaterial structure having finer grains. Blacking normally is to beavoided and a surface with finer grains obtained, if possible.

It is particularly advantageous according to the invention, if the lowerpiston is moved upwards to a mechanical stop element in its upmostposition, the upper end surface of the lower piston, when necessary,forming the lower surface of the object or objects to be molded. Also,the upper piston must be able to be moved upwards with a small counterpressure, in case the space in the mold and between the pistons iscompletely utilized and consequently allow the lower piston to reach itsuppermost position. Also, the actuation of the upper piston is to becompletely programmable, as regards time as well as speed.

Additional characteristics and advantages of the invention are set forthin the following description, reference being made to the accompanyingdrawings, which partly schematically show three non-limiting embodimentsof a device for press casting according to the present invention. Thedrawing show in detail in:

FIG. 1 the device according to the invention in its casting position,i.e. subsequent to the closing of the molds and the pouring in of liquidmetal but before the actual casting;

FIG. 2 the device according to FIG. 1 subsequent to the lowering of theupper piston into its holding position;

FIG. 3 the device according to FIGS. 1 and 2 subsequent to the movementof the lower piston and the filling of the mold with the upper pistondisplaced upwards carried out;

FIG. 4 the device according to FIGS. 1-3 in connection with a movementof the upper piston subsequent to the afterfeeding carried out;

FIG. 5 the device according to FIGS. 1-4, the mold being open and theupper piston being moved downwards completely and the product beingready to be ejected;

FIG. 6 the device according to FIGS. 1-5 subsequent to anejection/release of the molded product and a pulling inwards of theupper piston to the starting position for a new casting cycle;

FIGS. 7-12 a modified device according to the invention, the injectionsystem being displaced from the central area but for the restcorresponding to the device according to FIGS. 1-6; and

FIG. 13 a third embodiment according to the invention including anselectable system placed in the central area, and

FIG. 14 a fourth embodiment according to the invention with a mainlymodified feeding and pouring in of the liquid metal.

In the drawings a device according to the present invention in itsentirety is designated 1. This device comprises a machine frame 2, whichmainly comprises an upper base plate 3, from which machine bearers 4,e.g. four, project downwards, the lower ends of which support a lowermachine table 5, which rests on feet 6.

Upper base plate 3 supports hydraulically form cylinders 7, e.g. four,which are designed to open up and close a casting mold, which will bedescribed below, by means of an already known control system (notshown). From form cylinders 7 piston rods 9 project downwards throughbores 8 in the base plate. An upper machine table 10 is suspended at thelower ends of said piston rods 9 and is displaceable along machinebearers 4 by means of guide bushings 11.

Preferably in a central position on top of upper machine table 10 a yoke13 is disposed, at a distance from this machine table by means ofspacing rods 12, and this yoke 13 supports a cylinder 14 for an upperpiston, which cylinder projects upwards through an opening 15 in thebase plate, which opening is wide enough to also allow the passage ofthe yoke therethrough, in a position according to FIG. 3.

Through an opening 16 in yoke 13 a piston rod 17 with an upper piston 18projects downwards, which upper piston can be introduced into a fillingfunnel 21, mounted in upper machine table 10, which funnel leads to afeeding and guiding sleeve 19 in the upper mold part. Said sleeve leadsin its turn to a filling chamber 20 designed to fill a certain adjustedor predetermined amount of a liquid metal. Said filling chamber ismounted in the lower mold part och extends a small distance below thismold part, preferably having an enlarged diameter in order to allow aposition fixation between the lower mold part and the lower machinetable.

On the lower side of the upper machine table an upper mold part 22 isfastened, the lower side of which is provided with an upper mold cavity23.

On machine table 5 a lower mold part 24 rests, the upper side of whichin an analogous way is provided with a lower mold cavity 25.

Below machine table 5 a holder plate 27 is mounted by means of spacingrods 26, at a distance from the machine table, the holder plate 27 beingdesigned to hold ejection cylinders 28 mounted below it. Correspondingpiston rods 29 extend upwards through openings 30 in holder plate 27 andare designed to in unison support a bridge 31, which is displaceablymounted between machine table 5 and holder plate 27. Bridge 31 supportsin its turn e.g. 2-12 ejection rods 32, which extend upwards throughholes 33 in machine table 5 and are designed to, above this machinetable, support a common connection plate 34 with upper ejection plates35, ejection pins 37 projecting from these ejection plates into saidlower mold part 24 in guiding holes 36 in this mold part, by means ofwhich ejection pins 37 the casting, subsequent to its completed molding,can be ejected, when the mold has been opened up.

In the middle of and below bridge 31 a cylinder 38 of a lower piston ismounted, in which cylinder 38 a piston 39 is disposed, from which apiston rod 40 extends, which extends through openings 41 and 42respectively in bridge 31 and machine table 5 respectively in order to,with its free end, support a lower piston 43. Sleeve 19 and bushing 45may also be designed as one unit. Also, the bushings can be cooleddirectly or indirectly in a way known per se, not shown in detail inthis connection. However, cooling conduits are collectively designed bythe reference numerals 46 in FIG. 1.

Lower piston 43 and piston rod 40 suitably are cooled too, in which casefrom piston 39 a connection for a cooling equipment, not shown, mayextend downwards through cylinder 38.

The device now described can be used in the following way: FIG. 1 shows,as was mentioned above, this device in its molding position, the moldsbeing closed e.g. around an encased mold insert 48 to define a moldcavity 51. A certain adjusted amount of liquid metal, preferablybelonging to the group of non-iron-alloys, has already been filled inchamber 20 and is designated 49. The surface of the filled-in metalsuitably will reach a level which is slightly below the lower edge ofthe molding system.

In order to obtain this precise adjustment one not only start with apredetermined amount of metal but above all from the level of the lowerpiston, which according to the invention is adjusted to a desiredelevational position, which actually allows an exact computable fillingof metal, designed to carry out the casting in an optimal way and withthe least possible excess amount. The position of the lower piston isadjusted and indicated in a suitable way, e.g. by means of aprogrammable display panel in an electronic fashion. As appears from thefigure, the lower piston has a larger diameter, e.g. between 10 and300%, than the upper piston, which means that small piston movementsresult in larger volume changes. The ratio between the diameters of thelower piston and the length of the filling chamber ought to be between1:1 and 1:4, preferably about 1:2.5. Another important advantage of alower piston and then also a filling chamber with a larger diameter thanthe coaxially disposed upper piston and feeding and guiding sleeverespectively is that the downwardly running liquid metal is not allowedto splash into the molding system and be solidified in it prematurely.An additional advantage, thanks to these characteristics of theinvention is that the inner wall of the filling chamber, which isprovided with a lubricant, will not be sprayed with running metal, whichotherwise would result in an uneven distribution of the lubricant andrisks of biting and other drawbacks as a consequence. Now the fillingchamber wall will instead be-exposed to the liquid metal in a quieterway, which rises successively in the chamber, an even distribution ofthe lubricant being secured.

In addition to a liquid metal filling through a funnel and a sleeve intothe filling chamber, it is also possible to feed the liquid metal in alateral direction through a tube or a conduit or channel from a storagecontainer.

Subsequently the upper piston will be lowered according to FIG. 2 into apredetermined or programmed holding position, preferably roughly in themiddle of sleeve 19. This position is designed to allow a displacement,when a pressure is exerted, to a higher level and a displacementsubsequent to the metal feeding to an even higher level. In a practicalembodiment the upper piston is designed to, in said normal holdingposition, resist upwardly directed metal pressures of e.g. 1-20 barswithout a position alteration.

According to FIG. 3 the liquid metal has been fed into the moldingsystem by the lower piston, which thanks to its comparatively largediameter exerts an adequate or limited compression pressure, throughwhich the mold in a relatively quiet way is filled with metal, at thesame time as, thanks to the more compact design, a considerable amountof centrally stored heat is retained and transmitted to the center ofthe molding system. The movement of the lower piston is preferablyadjustable as regards the speed in the various sections, and it issuitably programmed. In the final phase of the upward movement of thelower piston the upper piston is influenced by a metal column, formedbelow it, its pressure finally exceeding the adjusted holding pressureand from then on resulting in a limited lifting of the upper piston toan upper holding position. All the time the upper piston continuesacting with an adjusted pressure on the metal column. The lower pistonfinally stops in an upper end position, preferably against a stationarystop element.

In the functional positions between FIGS. 3 and 4 the upper piston hasbeen activated, possibly with a time delay, to move further downwards,exercising an afterfeeding movement, which preferably is adjustable andprogrammable. During this movement the upper piston, which has a smallerdiameter, acts on the central, superheated area above the lower pistonand influences there the central metal portion, which is somewhat moremobile and substantially easier than the outer metal portion can becompressed, said central metal portion being compressed from within.Additional heat is thereby evolved, which enhances the flowcharacteristics of the metal and its workability. Thanks to the smallerdiameter of the upper piston the afterfeeding can be done efficiently,the porosity being lowered to a minimum, because the porosity mainly isdeveloped in the central thick-walled portions, which according to thedescription above are influenced primarily. Finally, the upper pistonwill stop in its lower holding position according to FIG. 4.

During the transition from the functional position according to FIG. 4to the position shown in FIG. 5 the upper piston continues first of allto exercise a high compression pressure against the metal column andconsequently increasingly projects out of the lifting upper mold part,until e.g. a stationary stop position according to FIG. 5 has beenreached, and simultaneously the upper mold part continues its movementupwards to the starting position shown in FIG. 5. When the metal hasbeen solidified in the mold, after e.g. 2-20 seconds, form cylinders 7are activated and in this way piston rods 9 will pull upper machinetable 10 upwards with upper mold part 22, and the upper side of thecasting will be free. In order to entirely eject the pressed piece 50from filling chamber 20 cylinder 14 of the upper piston remainsactivated and consequently it pushes the piston rod outwardssuccessively, when machine table 10 is moved upwards, the upper pistoncontinuously being pressed against the pressed piece, until the uppermachine table has reached its upper starting position according to FIG.3, in which the cylinder of the upper piston is activated in order topull the piston inwards to the position shown in FIG. 3. Of course,piston rod 17 can be pulled inwards somewhat earlier, e.g. as soon as arelative position has been reached, in which the pressed piece has leftthe filling chamber.

Cylinders 28 are activated now, which results in a lifting of bridge 31and finally in an activation of ejection pins 37, the casting beingfreed completely also from the lower mold part. The lower pistoncontributes to this also, since its cylinder is fastened to the bridge,resulting in a movement of also the lower piston besides the ejectionpins in relation to the lower mold part. When cylinders 28 have returnedto their starting position according to FIG. 1, cylinder 38 will beactivated, its piston 39 lifting the lower piston to the position shownin FIG. 3, which is the same position as the position shown in FIG. 2,and in this way a new molding cycle can be initiated.

The free end surface of the lower piston and preferably also its mantlesurface are according to a preferred embodiment of the inventionprovided with a ceramic coating in order to prevent a wear and/or obtaina limited heat insulation. The wall of the filling chamber can also bemade of a ceramic material. In this way the metal will be cooled to aminimal extent in the filling chamber, which is desirable, and at thesame time the thermal expansion of sleeve 19 and the lower pistonrespectively will also be relatively limited, which is desirable topreserve the basic tolerances. Also, the upper piston can be cooled inorder to achieve a quicker solidification and its mantle surface can, inorder to prevent a wear, be provided with a ceramic coating.

Whereas the transition time from one type of coating to another type isbetween 3 and 4 hours in a conventional press casting machine, thecorresponding transition time of a device or machine according to theinvention can be lowered to 10-15 minutes. This shows that substantialtime savings and rationalization gains are possible according to theinvention.

In accordance with the design shown in FIG. 14 the lower mold part canbe moved upwards and downwards by means of four cylinders 3', fastenedto the upper, movable machine plate. The ejection device has been movedfrom the lower machine plate to the upper one. The lower injectioncylinder is fastened to the lower machine table by means of a fasteningflange.

The advantage of this design is that the metal can be poured in to an aslow filling level as possible, a frothing and an oxidation being avoidedin this way or these effects at least being thoroughly counteracted.This is particularly true as regards aluminum alloys at elevatedtemperatures and during a low pressure molding. The definitely lowestfilling level, or pouring height is obtained, when the filling is donebelow the mold, as is depicted in the drawing. This could be done inthis embodiment by moving the lifting device for the lower mold part aswell as the ejection device.

This embodiment will function in the following way: During the fillingof the metal the lower piston is pulled downwards successively, and inthis way the metal surface will continuously appear adjacent the upperedge of the cylinder or the filling chamber, formed in this way.Subsequently the casting ladle will be removed and upper machine table10 moved downwards. Thanks to this movement the mold parts can be unitedby cylinders 3'. Cylinders 3' as well as the ejection device arefastened to the moveable machine plate. The recess in the middle of mold24 matches cylinder 44. Lower mold 24 now will rest on lower machineplate 5.

The casting is done in the same way as has been described above. Duringthe mold opening two things are activated simultaneously: a) an opening(cylinder 7) of upper machine plate 6'+4'+2'; b) an activation outwardsof cylinders 3' of the lower mold part, i.e. they work to retain lowermold part 24 on machine table 5 and in this way divide the mold intoparts 22 and 24. When the mold has been opened up sufficiently, the moldopening process is terminated and cylinders 3' and 7 cease working.

The casting has, thanks to the mold opening and a suitable moldconstruction, been retained in the upper mold part. A suitable device,designed in a way known per se, is introduced between the mold parts inorder to receive the casting, which is ejected by means of cylinders 28.The casting can now be removed by means of said device.

The lower mold part is now moved upwards to close the mold. Cylinders 3'are activated and in this way they will in unison pull the lower moldpart towards mold part 22. The machine cycle has been concluded and anew such cycle can be initiated with a filling of liquid metal etc.

I claim:
 1. A method of press casting in which a liquid metal (49) in afilling chamber (20) is displaced by a lower piston (43) mounted to alower machine table (5), under the influence of an upper piston (18)mounted in an upper machine table (10), the tables (5 and 10) support alower mold part (24) and an upper mold part (22) respectively, saidlower piston (43) is adjustable in the filling chamber (20) to cause theliquid metal to reach a desired level below a mold cavity (51) betweenthe mold parts (22, 24) and has a larger diameter than the upper piston,characterized by the steps of:a) lowering the upper piston, subsequentto pouring of the liquid metal into the filling chamber, to a desiredposition in a guiding and feeding sleeve (19) in the upper mold part; b)biasing the upper piston to this desired position; c) moving the lowerpiston upwards in the filling chamber to feed the liquid metal into themold cavity; d) applying sufficient upward feeding force by way of thelower piston and the liquid metal to the upper piston, once the moldcavity is full of the liquid metal, to move the upper piston upwardagainst the bias; e) allowing the liquid metal to solidify; f) loweringthe upper piston to a lower position; and g) opening the mold parts torelease the casting.
 2. A method according to claim 1, characterized byadjusting the lower piston to provide the desired level of liquid metalbelow the mold cavity.
 3. A method according to claim 2, characterizedin that the desired position of the upper piston is approximately at thecenter of the guiding and feeding sleeve in order to allow adisplacement due to upward pressure to a higher position and the bias issufficient to resist upwardly directed pressures of between 1 and 20bars before upward movement of the upper piston occurs.
 4. A methodaccording to claim 3, characterized by providing sufficient bias toensure a compression pressure of the liquid metal whereby the mold isfilled with the liquid metal.
 5. A method according to claim 3,characterized by providing sufficient bias to ensure a compressionpressure of the liquid metal whereby the mold is filled with the liquidmetal.
 6. A method of press casting in which a liquid metal (49) in afilling chamber (20) is displaced by a lower piston (43) mounted to alower machine table (5) to fill a mold cavity (23,25) defined by upperand lower mold parts (22,24), under the influence of an upper pistonmounted in an upper machine table, the lower piston (43) beingadjustable in the filling chamber (20) to cause the liquid metal toreach a desired level, characterized by the steps of:a) raising theupper and lower mold parts (22,24) to a raised position above the lowermachine table (5) and raising the lower piston (43) to a positionadjacent an upper edge of the filling chamber (20); b) pouring liquidmetal into the filling chamber (20) from a filling position below theupper and lower mold parts (22,24) and immediately adjacent the upperedge of the filling chamber (20), while simultaneously lowering thelower piston (43) maintaining an upper surface of the metal in thefilling chamber adjacent (20) the upper edge of the filling chamber (20)thereby keeping the pouring height as low as possible until the fillingchamber is filled to the desired level with liquid metal.
 7. A methodaccording to claim 6, further comprising the steps of:c) lowering theupper and lower mold parts (22,24) onto the lower machine table (5),subsequent to the pouring of the liquid metal into the filling chamber;d) lowering the upper piston, subsequent to the pouring of the liquidmetal into the filling chamber, to a desired position in a guidingsleeve (19) in the upper mold part; e) biasing the upper piston to thisdesired position; f) moving the lower piston upwards in the fillingchamber to feed the liquid metal into the mold cavity; g) applyingsufficient upward feeding force by way of the lower piston and theliquid metal to the upper piston, once the mold cavity is full of theliquid metal, to move the upper piston against the bias; h) allowing theliquid metal to solidify; i) lowering the upper piston to a lowerposition; and j) opening the mold parts to release the casting.
 8. Amethod according to claim 6, characterized in that the desired positionof the upper piston is approximately at the center of the guiding sleevein order to allow a displacement due to upward pressure to a higherposition and the bias is sufficient to resist upwardly directedpressures of between 1 and 20 bars before upward movement of the upperpiston occurs.
 9. A press casting machine comprising:a stationary baseplate (3) supporting an upper machine table (10) by hydraulic cylinders(7) for vertical movement relative to the base plate, the base plate andthe upper machine table being supported above a lower machine table; theupper machine table (10) having an upper mold part (22) rigidly mountedto a lower surface thereof, a lower mold part (24) supported, beneaththe upper mold part, by the upper machine table (10) (22) by hydrauliccylinders (3') for vertical movement relative thereto, and an upperpiston mounted in the upper machine table (1) that extends down into aguide sleeve (19) in the upper mold part that communicates with a moldcavity defined by the upper and lower mold parts (22,24); the lowermachine table (5) having a filling chamber (20) for receiving a liquidmetal (49) and a lower piston (43) mounted to the lower machine table(5) that extends upwards into the filling chamber (5) to fill the moldcavity (23,25) with the liquid metal under the influence of the upperpiston (18), the lower piston (43) being adjustable from a firstposition adjacent an upper edge of the filling chamber (20) to a secondposition at a desired level below the upper edge of the filling chamber(20) to maintain the lowest pouring height possible during pouring ofthe liquid metal and to measure the desired amount of liquid metal.